The present invention relates to a new method for etching an opening, and more precisely, to etching in a silicon plate for creating a nozzle opening. Such nozzle openings can be utilised in many areas, e.g. for various metering devices, in inkjet printers, etc. The invention enables accurate control of the dimensions of the nozzle opening. Through this, the amount of discharged fluid and the directional precision can be controlled very accurately.
It is previously known to use anisotropic etching for creating nozzle openings in silicon plates. The silicon plate is etched from one side so as to create a pyramid-shaped cavity. Then etching is performed from the other side so as to open the pyramid-shaped cavity at its tip. The etching is interrupted when the nozzle opening has reached the appropriate size.
With this previous technique there are certain problems. The surface area of the nozzle opening will be dependent on how deeply the etching is made from the other side. As a large number of nozzle openings are manufactured simultaneously from the same silicon plate, the etching must be interrupted at the same time for all the nozzles. Since the thickness of the silicon plate varies across its extension, the surface area of each individual nozzle opening will vary accordingly. The manufacturing precision will be unsatisfactory. Alternatively, a very carefully manufactured silicon plate must be used, which is costly. Furthermore, if the second side etching depth is large, the risk of the etching front having reached different depths in the plate at the time of breakthrough in the first nozzle also increases, which further increases the variation of the size of the nozzle openings.
The present invention solves this problem by, as before, etching a pyramid-shaped cavity from one side, but when the cavity has reached the depth where the intended opening is to be created, the etching is interrupted and a doping of the structured silicon surface is performed at this depth. After doping the silicon plate, the etching of the truncated pyramid cavities is continued, at least until the etching front has passed trough the doped region at the bottom of the truncated pyramid cavity. When the etching is then performed from the other side, the doped layer is given a potential in relation to a reference electrode in the etching solution, thereby protecting the doped layer from the etching solution. Since the bottom of the truncated pyramid is not doped, the etching from the other side will break through the bottom and open the nozzle. The etching can then be continued in order to free a conical nozzle rising over the silicon surface. It will be understood that this occurs independently of the thickness of the plate, as the position of the nozzle opening has already been established. Thereby, a high degree of accuracy is obtained for all nozzles across the entire silicon plate.
The present invention thus provides a method for etching an opening in a silicon plate.
According to the invention, one side of the silicon plate is protected by a protective layer, and a recess is made in the protective layer. Etching is made anisotropically through the recess so as to create a cavity in the shape of a truncated pyramid of a predetermined depth in the silicon plate. The cavity is doped so as to create a doped layer at the predetermined depth. The etching is then continued until the bottom surface of the cavity has passed the doped layer. Subsequently, etching is performed from the other side, while the etching stop in the doped layer is being activated, so as to free the nozzle opening at the other side.
The invention is defined in claim 1, whereas advantageous embodiments are stated by the dependent claims.